Field of the Invention
The present invention relates to a safety device having a simple structure for protecting a secondary battery from overcharge, over-voltage, over-current and over-heat.
Description of the Prior Art
Secondary batteries are rechargeable batteries including Ni—Cd batteries, Ni-MH batteries, and lithium ion batteries. Recently, as the lithium ion batteries have been spotlighted, studies and research have been actively carried out in relation to the lithium ion batteries. This is because the lithium ion batteries have advantages in that they have an energy density higher than that of the Ni—Cd batteries or the Ni-MH batteries. The lithium ion battery can be fabricated in a compact size with a light weight, so the lithium ion battery can be effectively utilized as a power source for portable electronic appliances, such as portable phones, camcorders or notebook computers. In addition, the lithium ion battery is extensively used as a power source for an electric vehicle, so the lithium ion battery has been currently spotlighted as a next-generation energy storage medium.
Although the lithium ion batteries have the above advantages, the lithium ion batteries present disadvantages that they are vulnerable to overcharge. If a secondary battery is not equipped with a safety device, accidental ignition or explosion of the secondary battery may occur due to the overcharge, thereby causing a dangerous accident or property loss. Therefore, it is very important for the secondary battery to prevent or restrict the overcharge or to solve problems derived from the overcharge.
For instance, when the lithium ion battery is subject to the overcharge, a negative reaction may increasingly occur between a cathode active material (e.g., LiCoO2) and an electrolyte of the lithium ion battery. Such a negative reaction destroys the structure of the cathode active material while causing an oxidation reaction of the electrolyte. In the meantime, lithium can be deposited on an anode active material consisting of graphite, etc. If the voltage applied to the secondary battery continuously rises even if the secondary battery has been overcharged, accidental ignition or explosion of the secondary battery may occur.
The above problem may become serious if the secondary battery is connected to a high voltage power source. For instance, if the lithium ion secondary battery is connected to a power source for a vehicle, 12V is applied in cases of automobiles, and 24V is applied in cases of freight cars because two power sources of 12V are connected in series. In this case, if an excessive voltage deviating from the standard for the secondary battery is suddenly applied to the secondary battery, a dangerous accident may occur, so that it is necessary to provide a safety device capable of effectively protecting the secondary battery from the excessive voltage.
For instance, Japanese Patent Unexamined Publication No. 2003-284237 discloses a safety device for a secondary battery including a zener diode and a thermal fuse thermally bonded to the zener diode. According to the above secondary battery having the above safety device, current flowing toward the zener diode suddenly increases when the secondary battery is subject to the over-charge voltage, so that power consumption of the zener diode is suddenly increased, thereby generating heat. As the zener diode generates heat, the thermal fuse connected to the zener diode is irreversibly cut off, thereby shutting off the current being applied to the secondary battery. According to Japanese Patent Unexamined Publication No. 2003-284237, the breakdown voltage of the zener diode is employed in order to disconnect the thermal fuse when the secondary battery is subject to the over-charge voltage. However, if the breakdown voltage of the zener diode is slightly higher than a maximum charge voltage of the secondary battery, the zener diode may have the leakage current when the secondary battery is normally operated although the overcharge of the secondary battery can be prevented.
It is generally known in the art that the zener diode has the leakage current under a predetermined voltage lower than the breakdown voltage of the zener diode by 1V or less. Thus, if the leakage current is generated from elements connected to the cathode and the anode of the secondary battery, the secondary battery may be self-discharged, so that the operating time and lifetime of the secondary battery may be reduced after the secondary battery has been charged.
If a zener diode, which does not cause the leakage current under the charge voltage of the secondary battery, is used for the secondary battery, the current cannot be sufficiently discharged when the secondary battery is subject to the overcharge. In addition, when a high current is applied to the zener diode, the zener diode is broken so that the zener diode may not play its original role. Even if the voltage rises, the resistance is so high that the current cannot flow through the zener diode.